As explained hereinabove, it is desirable to have a compound of SBR or other diene-containing synthetic rubbers with high green strength and good building tack. Through the years, increasing the green strength of SBR has been attempted by increasing molecular weight, partial crosslinking, modifying the polymer structure and the addition of other polymers.
Regarding the addition of other polymers, U.S. Pat. No. 3,798,190 discloses rubber compositions of high green strength comprising blends of styrene-butadiene rubber, with or without natural rubber, and a hydrogenated random copolymer of styrene-butadiene.
A recent U.S. Pat. No. 4,254,013 discloses an attempt to improve the green strength of elastomeric blends by the addition of an unsaturated acid to the synthetic component of a natural or synthetic cis-1,4-polyisoprene and synthetic elastomer composition whereby the green strength is increased by the formation of ionic bonds. Despite the improvement of green strength by the addition of the unsaturated acid, the patent still recommends a range of 50 to about 90 parts of natural rubber for those blends which exhibit improved green strength.
The preparation of a synthetic diene-containing rubber having improved green strength has been set forth in copending Ser. No. 132,736, commonly owned by the assignee of record herein. There, it has been disclosed that a terpolymer comprising styrene and butadiene, or similar monomers, and an N-(alkoxymethyl)acrylamide exhibits improved green strength over conventional SBRs.
Prior art considered during prosecution of the aforementioned patent application such as U.S. Pat. No. 3,037,963 disclosed the preparation of aldehyde-modified carboxylic acid amide resinous materials comprising in specific instances, styrene, butadiene and N-(alkoxymethyl)acrylamide. The materials were said to be useful as coating compositions.
Thus, the art considered herein has not provided a blend of natural and synthetic rubbers, having the green strength, building tack and processability of natural rubber. Merely by substituting various synthetic rubbers to reduce the natural rubber content to about 30 percent by weight, based upon the total weight of the rubber, it has not been possible to provide rubber compounds suitable for building tires.